That is the TYPE of change you need to make, the above will give you a button right below System in the xTV skin, it will also look really bad if you have a disc in, and the animation will be off, but above is how you would need to do it, I tested it, and it works fine.
You press down from System, the Apple Movie Trailers gets highlighted, when you press select, Apple trailers opens right up.
Use the above as a idea, and make it look the way you want it.